Electrical systems in residential, commercial and industrial applications usually include a panelboard for receiving electrical power from a utility source. The electrical power is then delivered from the panelboard to designated branch circuits supplying one or more loads. Typically, various types of protective devices are connected to the branch circuits. The protective devices may be mounted within the panelboard or external to the panelboard.
Circuit breakers are a well known type of protective device which are designed to trip open and interrupt an electric circuit in response to detecting overloads and short circuits. Overload protection is often provided by a thermal element which, when heated by the increased current, will cause the circuit breaker to trip and interrupt the power. This can occur when too many loads draw power from the same branch circuit at the same time, or when a single load draws more power than the branch circuit is designed to carry. Short circuit protection is often provided by an electromagnetic or "magnetic" coil element that trips the breaker when a high current flows through the coil. Additionally, many circuit breakers include ground fault interruption (GFI) circuitry to protect against ground faults which are defined as current flowing from a hot conductor to ground.
In many circuit breaker designs, the thermal element comprises a bimetallic element which supply the mechanical energy or work to delatch the circuit breaker in the event of a current overload condition. That is, the bimetallic element must work against the latching force of the circuit breaker to unlatch and open the circuit breaker. The force required to open the circuit breaker is dependent on the latch load and the surface friction of the latch. The latch load is a function of the force required at the circuit breaker contacts. Therefore, the bimetallic element must be sized and designed to supply the required force and deflection to delatch the circuit breaker mechanism.
Since the latch surface friction and latch load will vary from one type of circuit breaker to another, as well as between circuit breakers of the same type, a number of manufacturing problems arise. For example, a number of different bimetallic elements specifically designed for different types of circuit breakers must be designed and manufactured. Moreover, calibration of circuit breakers of the same type is complicated by the mechanical tolerance differences between individual units. Even within a given circuit breaker, the latch force will vary somewhat over the service life of the unit, since each time the circuit breaker is tripped and relatched, some variation in these mechanical forces will occur. Also, the latch surface is often contaminated over time with dust and other debris.
The electromagnetic element for short circuit protection usually uses a so-called magnetic coil. However, in many units the thermal and magnetic trip mechanisms are highly integrated, such that normal operation of the thermal trip system will directly affect the operation of a magnetic trip system, although usually not vice-versa. Thus, for example, ambient compensation for the thermal trip system will change the magnetic trip level. More particularly, as the ambient temperature increases or decreases, the magnetic gap is increased or decreased due to the thermal adjustments of a thermal compensator which is a part of the thermal trip system.
The bimetallic element used in the thermal overload protection or trip system usually is required to be a cantilevered type of bimetallic element. Snap-acting bimetallic elements have been used in relatively lower power circuit breakers. One of the main advantages of using a snap-acting bimetallic element is the capability for thermal calibration during the bimetal manufacturing process in such snap action elements. This can permit the manufacture of a thermal trip system for a circuit breaker which does not require calibration after assembly, therefore reducing production costs. However, due to the limited amount of deflection available in a snap-acting bimetallic element, use of such elements has heretofore been limited to relatively low current, low voltage applications. Because of the relatively small amount of the deflection, the air gap provided in a snap acting bimetallic element has been considered not useable in circuit breakers of higher current, voltage and interrupting ratings.
In the above-mentioned present designs which integrate thermal and magnetic trip units, stamped and formed parts have been required by this integration of the elements. However, we have found that design of the magnetic trip unit alone, without the requirement of integrating thermal trip elements allows for the elimination of stamped and formed parts and the use of molded features. Molded features are typically more precise and repeatable than stamped and formed parts. The use of molded features in the magnetic unit, and the use of a snap-acting bimetallic element in the thermal trip unit would also tend to improve the durability, e.g. shock resistance, of the trip systems. The neutral line is usually not brought in to the breaker in conventional designs employing thermal and magnetic trip elements.
Electronic circuit breaker arrangements are also known and used. In such electronic circuit breakers, a sensing element such as a current transformer coil senses a current flowing through a wire which passes through the sensor coil. An electronic circuit is responsive to the signal produced by the sensor coil or other sensor element for tripping the circuit breaker if this signal has certain predetermined characteristics. Such electronic circuit breakers may be used not only for overcurrent or short circuit protection, but also for arcing fault interruption. That is, the electronic circuit can be arranged to sense characteristics of the incoming sensor signal indicative of the existence of an arcing fault, and produce a trip signal for tripping the circuit breaker in response to such signal conditions.